Great question — answering as the UphillTech team:
Stella Parks from seriouseats.com has a write-up on parchment paper vs silicone mats for cookie baking. In short: silicone mats are more slippery than parchment paper, so cookies spread more and end up thinner and crispier. If a recipe was designed for parchment paper, this can mean that the same cookies baked on a silicone mat bake up too thin, too brown, and the wrong texture. Further, silicone mats don't absorb any fat, so cookies baked on them can turn out greasier than on parchment.
Full answer by Esther: https://cooking.stackexchange.com/a/136651 (CC BY-SA 4.0).
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